fis·​sion | \ˈfi-shᵊn, -zhᵊn\

Definition of fission 

(Entry 1 of 2)

1 : a splitting or breaking up into parts

2 : reproduction by spontaneous division of the body into two or more parts each of which grows into a complete organism

3 : the splitting of an atomic nucleus resulting in the release of large amounts of energy


fissioned; fissioning; fissions

Definition of fission (Entry 2 of 2)

intransitive verb

: to undergo fission

transitive verb

: to cause to undergo fission

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Other Words from fission


fissional \ˈfi-​shə-​nəl, -​zhə-​ \ adjective

Examples of fission in a Sentence

Recent Examples on the Web: Noun

Males always disperse for other groups in this fission-fusion society, and about one-third of femalees will go off to other prides. National Geographic, "Failure to Launch: These Animals Stay With Mom For Years," 12 May 2018 Advanced fission is unlikely to commercialize in the next few decades. David Roberts, Vox, "The US is rapidly losing nuclear power. That’s profoundly concerning for climate change.," 11 July 2018 Berg’s assignment is to determine whether the scientist is making a fission bomb for the Nazis. Robert Rodat, New York Times, "Review: ‘The Catcher Was a Spy’ Shares a Peculiar History Lesson," 21 June 2018 When there are enough fissile atoms close together — a quantity known as critical mass — the particles ejected by fission can strike other fissile atoms, triggering more atoms to split apart and so on. Umair Irfan, Vox, "Uranium enrichment will be a key point in talks with North Korea. Here’s what it is.," 11 June 2018 The fission of uranium also produces plutonium, which can be used in nuclear weapons. Kenneth Chang, New York Times, "How to Spot a Nuclear Bomb Program? Look for Ghostly Particles," 27 Mar. 2018 Forthwith, the issues multiply, as if by fission: What will people think? John Anderson, WSJ, "‘A Kid Like Jake’ Review: Cutthroat Kindergartens," 31 May 2018 The Kilopower project uses nuclear fission — the process in which an atom is split, releasing a tremendous amount of heat energy. Rob Nikolewski,, "NASA looks to send a small nuclear reactor to the moon and Mars," 23 May 2018 Lacking cooling water, the fuel—including the radioactive fission products—heated up and began to melt. Andrew Karam, Popular Mechanics, "Five Years Later, Cutting Through the Fukushima Myths," 11 Mar. 2016

Recent Examples on the Web: Verb

The point is that by analyzing the array of radioactive fission products – those that can be captured and analyzed – nuclear forensic specialists can start to figure out if the atoms that fissioned were U-235, Pu-239, or (in some cases) U-238. Andrew Karam, Popular Mechanics, "Nuclear Forensics: How Scientists Can Tell Whether North Korea Is Lying About Its Bomb Test," 7 Jan. 2016 Anyone can see that fissioning uranium 235 or plutonium can be used to generate electricity. Scientific American, "Readers Respond to the January 2018 Issue," 1 Jan. 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'fission.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

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First Known Use of fission


circa 1617, in the meaning defined at sense 1


1929, in the meaning defined at intransitive sense

History and Etymology for fission


borrowed from Latin fissiōn-, fissiō, from fid-, variant stem of findere "to split, cleave" + -tiōn-, -tiō, suffix of verbal action — more at bite entry 1


derivative of fission entry 1

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Statistics for fission

Last Updated

19 Oct 2018

Look-up Popularity

Time Traveler for fission

The first known use of fission was circa 1617

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More Definitions for fission



English Language Learners Definition of fission

physics : a process in which the nucleus of a heavy atom is split apart

biology : a kind of reproduction in which a cell or body divides into two or more parts and each part grows into a whole new individual


fis·​sion | \ˈfi-shən \

Kids Definition of fission

1 : a method of reproduction in which a living cell or body divides into two or more parts each of which grows into a whole new individual

2 : the process of splitting an atomic nucleus with the release of large amounts of energy


fis·​sion | \ˈfish-ən also ˈfizh-\

Medical Definition of fission 

1 : a method of reproduction in which a living cell or body divides into two or more parts each of which grows into a whole new individual

2a : cleavage sense 2

b : the splitting of an atomic nucleus resulting in the release of large amounts of energy

called also nuclear fission

Other Words from fission

fission verb fissioned; fissioning\ˈfish-​(ə-​)niŋ also ˈfizh-​ \
fissional \ˈfish-​ən-​ᵊl also ˈfizh-​ \ adjective

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More from Merriam-Webster on fission

Rhyming Dictionary: Words that rhyme with fission

Spanish Central: Translation of fission

Nglish: Translation of fission for Spanish Speakers

Britannica English: Translation of fission for Arabic Speakers Encyclopedia article about fission

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